The most common uses of a pH meter are in the measurement of pH, and in chemical analyses where they are used to measure the end point of an acid-base titration. All pH meters are based on a general method of measuring the potential difference between an indicator electrode, which responds to the activity of hydrogen ion in solution, and a reference electrode whose potential remains constant throughout the course of the potentiometric measurement. This potential difference produced is proportional to the hydrogen ion activity of the sample solution, thus enabling the determination of solution pH.
The most widely used and convenient pH meter utilizes a glass membrane electrode. Membrane electrodes are those electrodes that measure the potential difference that develops across a thin glass membrane separating two solutions having different hydrogen ion concentrations. A known glass electrode 2 typically consists of an indicator electrode 4 and a reference electrode 6 immersed in a solution 8 whose pH is to be measured. (See FIG. 1.) These electrodes are connected via external leads 10 and 12 to separate terminals of a potential measuring device such as a potentiometer.
The indicator electrode 4 is typically a thin-walled glass bulb 16 containing a solution having a constant pH and a platinum wire 18 immersed in the solution. The indicator electrode solution 14 is called a buffer solution, having a known pH that does not vary with the addition of small amounts of an acid or a base.
It is also necessary to employ a reference electrode 6 to maintain an essentially constant and reproducible potential in the presence of small currents. Ideally, this electrode is entirely insensitive to the solution under study. This is distinguishable from the indicator electrode 4 whose response is dependent upon the analyte concentration.
A well-known reference electrode that is often described in the prior art may be a calomel electrode 6. An example of a standard calomel reference electrode is shown in FIG. 2. A calomel electrode utilizes a mercury chloride paste 26 contained in an inner tube 24. An outer tube 20 of the calomel electrode is typically filled with a saturated solution 22 of potassium chloride.
The potential of the calomel electrode varies directly with the chloride concentration of potassium chloride. The design of the calomel electrode may be represented as follows: EQU .parallel.Hg.sub.2 Cl.sub.2 (sat'd), KCl(cM).vertline.Hg
The electrode reaction of mercurous chloride is: ##STR1##
Another reference electrode 6 analogous to the calomel electrode is the silver/silver chloride electrode having a wire electrode 19 made of silver immersed in a solution of potassium chloride that has been saturated with silver chloride. (See FIG. 1.)
The shorthand cell representation of the silver/silver chloride electrode is: EQU .parallel.AgCl(sat'd),KCl(cM).vertline.Ag
The electrode reaction of silver chloride is: ##STR2##
The body of both types of reference electrodes consists of an outer glass tube 20 typically filled with a solution 22 of saturated potassium chloride or other similar solution containing a hard cation component such as sodium. (See FIG. 1.) An inner tube 24 contains a saturated solution 26 of mercurous chloride or silver chloride and saturated potassium chloride. The inner tube 24 has a small opening 28 to allow for passage of ions between the two electrode compartments. The outer tube 20 is immersed in the sample solution 8, and contacts the sample solution by means of a fritted disk 30 or a porous fiber sealed in the end of the outer tubing. (See FIG. 1.) Instead of a fritted disk 30 or porous fiber, the outer tube 20 may have a ground glass sleeve 32 to achieve electrical contact with the sample solution. (See FIG. 2.)
One problem encountered with the reference electrodes in the prior art is that constituents of a sample solution 8 whose pH is to be measured, may have a strong affinity for cations. These cation-sensitive compounds may form a precipitate at the phase boundary 34 between the fritted glass channel, filled with the potassium chloride and the sample solution. (See FIG. 1.) This precipitate disrupts ion flow through this reference electrode thereby leading to electrode instability. This reaction occurs with sample solutions that interact strongly with hard cations, such as potassium or sodium, and form insoluble salts. Solutions containing hard cations such as potassium and sodium are generally used as electrolyte solutions in reference electrodes. For example, a concentrated KCl solution is used as solution 22. (See FIGS. 1 and 2.) The result of such a reaction is that the pH electrode becomes contaminated and unstable. The instability causes large errors in the pH reading, many as large as 1 pH unit, and often greater. For this reason, it is desirable to use a reference electrode solution containing an electrolyte compound, and to provide a method of using the same, such that the electrolyte comprises a non-interacting (soft) cationic compound that does not react with the sample solution to form precipitates. As will be discussed below, compounds such as quaternary ammonium salts, and quaternary phosphonium salts, when used in a reference electrode solution, serve as a replacement electrolyte without forming precipitates with constituents in the sample solution.
Ammonium salts are commonly used in the prior art for a variety of chemical applications. Noteworthy uses of these compounds include U.S. Pat. No. 5,116,481, issued to Ozawa et al. Ozawa is directed to an anion-selective sensitive film in an anion selective electrode for measuring the concentration of inorganic anions, such as chloride ions contained in body fluids. The anion-selective sensitive film contains an anion sensitive substance, such as quaternary onium salts, including quaternary ammonium salts and quaternary phosphonium salts. These anion selective substances are supported by a polymeric film containing a high polymer and plasticizer. (Column 2, lines 24-27.) Of the quaternary onium salts, phosphonium salts are preferred for their high selectivity for ceratin ions including chlorate, thiocyanate, iodide and nitrate ions and their low selectivity for other ions such as hydroxyl or fluoride ions. (Column 5, lines 17-66.) This high selectivity ability reportedly produces accurate anion measurement with longer electrode life. (Column 6, lines 10-19.) The use of ammonium salts and phosphonium salts in the present invention is readily distinguishable from their use in Ozawa. Unlike the disclosure in Ozawa, the present invention does not use quaternary ammonium salts and quaternary phosphonium salts to form an anion sensitive film or membrane. Rather, in the present invention, quaternary ammonium salts and quaternary phosphonium salts are used in a reference electrode solution in a pH electrode. Further, the reference electrode solution of the present invention is used to accurately measure the pH of a sample solution, not to measure anion concentration.
The present invention is also distinguishable from U.S. Pat. No. 5,066,373 issued to Levy et al. Levy discloses an electrode system for monitoring and controlling the pH of phenol-acetone streams used to produce phenol and to maximize and isolate useful by-products of cumene hydroperoxide, a compound used to create phenol. Levy discloses ammonium salts dissolved in a solution of phenol, acetone, and water to form an electrode solution. (Column 2, lines 35-40.)
Levy discloses the preferred use of ammonium salts in an electrolyte solution to obviate the necessity of adding an external solvent such as water, to create a time invariant voltage and to decrease contamination. (Column 2, lines 16-45.) This consistent voltage is used to continuously monitor the acidity of the neutralized cleavage mixture of phenol-acetone-cumene process streams.
The use of ammonium salts and phosphonium salts in the present invention is dissimilar to the use of ammonium salts in Levy. Unlike their use in Levy, the present invention utilizes ammonium and phosphonium salts as a replacement aqueous electrolyte to prevent the formation of precipitates with cation-sensitive constituents in sample solutions. In contrast, Levy discloses the use of ammonium salts in combination with phenol, acetone, and water to create a soluble and stable electrolyte solution. This solution is disclosed as effective in measuring the acidity of a mixture used to create phenol, not to measure the pH of solutions containing cation-sensitive compounds.